When editing a Software Product (from Manage > Software or Manage > Software Catalog), after picking a product from the Installed Products list and then clicking on its Manage Licenses button, the Add License Purchase button is disabled and cannot be used.
Ensure that the "Licensable product" checkbox is checked (enabled) to allow licenses to be added. If this is already added but the Add License Purchase button is still not enabled for use, it is possible that this is occurring due to data issues. If this works on other Software Products, then something with the record for the failing on is the issue. Try manually adding the license in the Asset Management user interface by going to Home > Service and Asset Management > Software Licensing.
If Asset Management Solution is not installed, this section will also be disabled and unusable, with the warning "Software License Management [i.e., Asset Management] not installed" will appear. Install Asset Management to utilize this function.
Beyond this, it's not known what may actually cause this. Some possibilites are: